getpass: Don't fail if stdin is not a tty

We don't need to manipulate the tty state (such as turning off
echo) when prompting for passwords if we're not reading from a tty.

Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>

Autobuild-User(master): Alexander Bokovoy <ab@samba.org>
Autobuild-Date(master): Fri Apr  5 07:34:37 CEST 2013 on sn-devel-104
doc: Document performance impact of "hide unxx" parameters

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: David Disseldorp <ddiss@samba.org>

Autobuild-User(master): David Disseldorp <ddiss@samba.org>
Autobuild-Date(master): Fri Apr  5 18:40:58 CEST 2013 on sn-devel-104